Warm Temperatures Activate TRPV4 in Mouse 308 Keratinocytes

Abstract

Mammalian survival requires constant monitoring of environmental and body temperature. Recently, several members of the transient receptor potential vanilloid (TRPV) subfamily of ion channels have been identified that can be gated by increases in temperature into the warm (TRPV3 and TRPV4) or painfully hot (TRPV1 and TRPV2) range. In rodents, TRPV3 and TRPV4 proteins have not been detected in sensory neurons but are highly expressed in skin epidermal keratinocytes. Here, we show that in response to warm temperatures (>32°C), the mouse 308 keratinocyte cell line exhibits nonselective transmembrane cationic currents and Ca 2؉ influx. Both TRPV3 and TRPV4 are expressed in 308 cells. However, the warmth-evoked responses we observe most closely resemble those mediated by recombinant TRPV4 on the basis of their electrophysiological properties and sensitivity to osmolarity and the phorbol ester, 4␣-phorbol-12,13-didecanoate. Together, these data support the notion that keratinocytes are capable of detecting modest temperature elevations, strongly suggest that TRPV4 participates in these responses, and define a system for the cell biological analysis of warmth transduction.The perception of ambient temperature is a physiological process critical to the maintenance of body temperature and the avoidance of painful or dangerous thermal extremes. Since the identification of the heat-sensing ion channel TRPV1 (1), 1 which is activated by temperatures above 42°C, there has been a significant focus on potential thermosensory functions for this and other ion channels of the transient receptor potential family. TRPV2 (2) and TRPV3 (3-5) were first described as heat transducers operative at very hot (Ͼ52°C) and moderately warm (Ͼ34°C) temperatures, respectively. TRPV4, which was originally identified as an osmosensory ion channel (6 -9), can also be activated by warm temperatures (Ͼ34°C) (10, 11). In addition, two TRP proteins outside of the TRPV subfamily, TRPM8 (12, 13) and ANKTM1 (ankyrin repeat/transmembrane-containing ion channel) (14), have been identified as cold-activated ion channels expressed in sensory neurons.In mammals, the skin is extremely important for the transduction of thermal information and its transmission to the central nervous system. Cutaneous thermosensation has been largely attributed to the sensory nerves that innervate the dermal and epidermal layers of the skin. Recent reports, however, have suggested the possibility that other skin components, most notably keratinocytes, might also participate in temperature sensation. TRPV1 and TRPV2 are highly expressed in distinct subsets of sensory neurons (2). In humans, TRPV3 is also expressed in sensory neurons (5). In contrast, attempts to detect TRPV4 (9, 10) and TRPV3 (3) at the protein level in rodent sensory neurons have been unsuccessful. Rather, immunohistochemical studies of mouse and rat skin have revealed that keratinocytes exhibit the greatest degree of cutaneous TRPV3 (3) and TRPV4 (10) expression. In addition, although peripheral sensory neurons ...

Phosphatidylinositol-4,5-biphosphate-dependent rearrangement of TRPV4 cytosolic tails enables channel activation by physiological stimuli

“…S7A). In the absence of PIP 2 , and consistent with previous reports (8,21,43), no significant change in channel activity was elicited by heat (Fig. 3E).…”

“…The TRPV4 temperature coefficient (Q 10 ) obtained from excised patches containing TRPV4-WT in the presence of PIP 2 was 21 ± 5 (n = 3) (Fig. S7B), consistent with values obtained from previous TRPV4 whole-cell recordings (21,43). Considered together, these experiments confirm that PIP 2 is required for TRPV4 activation by physiological stimuli, most likely through action as an allosteric modulator.…”

Cell swelling, heat, and chemical agonists use distinct pathways for the activation of the cation channel TRPV4

“…This determination would require the direct measurement of 4␣-PDD binding to membranes containing WT and mutant TRPV4, similar to the demonstration of the capsaicin binding site in TRPV1 (34). In contrast to TRPV1, which reportedly is activated directly by heat, recent findings indicate that heat activates TRPV4 in cell-attached patches but not in cell-free inside-out patches (6,35). This finding might indicate that heat activation of TRPV4 requires production of a soluble TRPV4-activating molecule.…”

The TRPV3 mutation associated with the hairless phenotype in rodents is constitutively active

“…Amongst all mammalian TRP channels, only TRPV3 and TRPV4 have been shown to be highly expressed and functional in skin keratinocytes [4][5][6][7][8]. Most studies have focused on the sensory functions of these channels, primarily the sensation of warm and hot temperatures.…”